Massachusetts Senator John F. Kerry, Bush's Democratic opponent in 2004, also released part of his wife's tax return after his campaign came under pressure to disclose details of her personal fortune, which was helping to fund his bid. Kerry, who had released his own full return, released portions of the tax return of Teresa Heinz Kerry -- heiress to a fortune estimated at $500 million -- three weeks before the election.

Senator John McCain of Arizona, the 2008 Republican presidential nominee whose wife, Cindy, is also an heiress, released a portion of her tax return a few weeks before that year's election.

Paul Caron, a tax law professor at the University of Cincinnati College of Law, said a refusal by Romney as a presidential nominee to make his income tax return public likely would "become a narrative that sets him up really badly" in the campaign and he'd have to back down eventually.

"It's sort of unsustainable -- it flies in the face of an awful lot of history," said Caron, who edits the TaxProf Blog. "As one of the wealthier folks to ever run, there's probably a heck of a lot there that he's not excited about having analysts be able to comb over, so it makes a lot of sense that he's wary of doing it."

A $10,000 Bet

Romney has drawn criticism for appearing out of touch with ordinary Americans because of his affluence, particularly after a rhetorical flourish he used in a Dec. 10 televised debate in which he offered Texas Governor Rick Perry a $10,000 wager while defending his health-care record.

Obama's campaign and Romney's Republican primary opponents ridiculed him for the bet. He has also come under scrutiny for the way he earned his fortune -- as head of the private equity firm Bain Capital LLC, where he bought and sold companies to make money for investors and sometimes eliminated jobs in the process. Former House Speaker Newt Gingrich of Georgia suggested last week that Romney might return the money he made "bankrupting companies and laying off employees over his years at Bain."

'Gekko' Photo

Romney said on Dec. 18 that if he wins the nomination, he knows that a photo in which he appears grinning with other Bain executives posing with cash spilling out of their hands, jackets and mouths will come back to haunt him. Some Democrats call the 1980s-era image the "Gordon Gekko" photo, referring to the antihero in the 1987 film "Wall Street" whose character is the personification of greed run amok.

Romney said on "Fox News Sunday" that if he is the nominee, he expects that free enterprise will be "on trial" in his contest with Obama.